The avian representative Gallus gallinaceous has proven to be a useful model for the study of immune system development and function. The chicken TCR gene loci are similar to those in mammals but contain relatively few V, D, J and C gene segments. The hypothesis that gamma/delta T-lymphocytes, including the intestinal CD8 alpha/alpha subpopulation are generated in the thymus and subsequently interact with alpha/beta T-cells to modulate immune responses in peripheral lymphoid tissues will be tested in this avian model. The initial experiments aim to complete the definition of the recently identified gamma and delta TCR loci and to produce V-region specific antibodies, and the resultant probes and antibodies will be employed to obtain a composite picture of the rearrangement and expression of TCR alpha, beta, gamma and delta genes during T-cell development. To further address the controversial issue of an extrathymic origin for T-cells in mucosal epithelium, a search for in situ TCR gene rearrangement will be conducted. The formation of DNA deletion circles and double-strand DNA breaks during TCR rearrangement and expression of the recombination activation genes will be analyzed in central and peripheral lymphoid tissues. The stability of TCR V(D)J deletion circles in thymus-derived T-cells will be evaluated in thymectomized birds employing a novel thymocyte-specific marker (CT1) to identify immature T-cells including recent thymic emigrants to the periphery. The idea that gamma/delta T-cells can modulate the function of alpha/beta T-cells will be examined in graft-versus-host (GVH) reactions, taking advantage of the ready accessibility of the chick embryo, the relative abundance of cells and prior definition of the cellular evolution in splenic GVH lesions. The TCR repertoire of the alloreactive alpha/beta T-cells of donor origin and the interactive gamma/delta T-cells of host origin will be defined in GVH foci on chorioallantoic membrane. These studies should yield information on fundamental principles governing the development of gamma/delta and alpha/beta T-cells and their functional interactions, findings that could have broad clinical implication.